1. Field of the Invention
The present invention relates to a thermally assisted magnetic head for writing signals by a thermally assisted magnetic recording scheme, a head gimbal assembly (HGA) equipped with the thermally assisted magnetic head, and a hard disk drive equipped with the HGA.
2. Related Background Art
As hard disk drives have been increasing their recording density, thin-film magnetic heads have been required to further improve their performances. As the thin-film magnetic heads, composite thin-film magnetic heads having a structure in which a magnetization detecting device such as magnetoresistive (MR) device and a magnetic recording device such as electromagnetic coil device are laminated have widely been in use. These devices read/write data signals from/onto magnetic disks which are magnetic recording media.
In general, a magnetic recording medium is a sort of discontinuous body in which magnetic fine particles gather, while each magnetic fine particle has a single-domain structure. Here, one recording bit is constituted by a plurality of magnetic the particles. For enhancing the recording density, the magnetic fine particles must be made smaller, so as to reduce irregularities in boundaries of recording bits. When the magnetic fine particles are made smaller, however, deteriorations in thermal stability of magnetization due to the reduction in volume become problematic.
An index of thermal stability in magnetization is given by KUV/kBT. Here, KU is the magnetic anisotropy energy of the magnetic fine particle, V is the volume of one magnetic fine particle, kB is the Boltzmann constant, and T is the absolute temperature. Making the magnetic fine particles smaller just reduces V, so that KUV/kBT becomes smaller, thereby deteriorating the thermal stability. As measures against this problem, KU may be increased at the same time. The increase in KU, however, raises the coercivity of the recording medium. By contrast, the writing magnetic field intensity by a magnetic head is substantially determined by the saturated magnetic flux density of a soft magnetic material constituting a magnetic pole within the head. Therefore, the writing becomes impossible when the coercivity exceeds a permissible value determined by the limit of writing magnetic field intensity.
As a method for overcoming such a problem of thermal stability in magnetization, a so-called thermally assisted magnetic recording scheme has been proposed, which performs writing with a lower coercivity by applying heat to a recording medium immediately before exerting the writing magnetic field, while using a magnetic material having a large KU. This scheme is roughly divided into a magnetically dominant recording scheme and an optically dominant recording scheme. The magnetically dominant recording scheme employs an electromagnetic coil device as a main part for writing, while the radiation diameter of light is larger than the track width (recording width). On the other hand, the optically dominant recording scheme employs a light radiation part as a main part for writing, while the radiation diameter is substantially the same as the track width (recording width). Namely, the magnetically dominant recording scheme provides a magnetic field with a spatial resolution, whereas the optically dominant recording scheme provides light with a spatial resolution.
As such a thermally assisted magnetic bead, Japanese Patent Application Laid-Open No. 2005-190655 discloses one in which an optical waveguide is provided near an electromagnetic coil device. In this structure, light emitted from a light-emitting device is introduced into the optical waveguide and then caused to emit from a light exit surface of the optical waveguide within a medium-opposing surface, so as to heat a magnetic recording medium locally. Subsequently, the electromagnetic coil device applies a writing magnetic field to a local area of the magnetic recording medium where the coercivity is lowered by the heating.
Also, IEEE Trans. Magn. Vol. 41, p. 2817 (2005) discloses a thermally assisted magnetic head utilizing a U-shaped near-field light generating part formed on a quartz slider. In his structure, a U-shaped curved portion of the near-field light generating part is irradiated with laser light, so as to generate near-field light thereby locally heating a magnetic recording medium. Subsequently, a current is caused to flow through the near-field light generating part, and writing is effected by an inductive magnetic field generated from the curved portion.